1. Field of the Invention
This invention generally relates to Group VA-doped metal chalcogenides and, more particularly, to processes for forming antimony-doped metal chalcogenides using solutions of metal precursors.
2. Description of the Related Art
Metal and mixed-metal chalcogenides represent important classes of semiconductor materials for electronic and photovoltaic (PV) applications. In particular, copper indium gallium diselenide (CuIn1-xGaxSe2 or CIGS) has emerged as a promising alternative to other existing thin-film technologies. Overall, CIGS exhibits a direct and tunable energy band gap, high optical absorption coefficient in the visible to near-infrared (NIR) spectrum and has demonstrated power conversion efficiencies (PCEs)>20%. Conventional CIGS fabrication (vacuum) processes typically involve either sequential or co-evaporation (or sputtering) of copper (Cu), indium (In), and gallium (Ga) metal onto a substrate followed by annealing in an atmosphere containing a selenium source to provide the final CIGS absorber layer.
In contrast to vacuum approaches, which create an environment to control variables such as the gases introduced and pressure, non-vacuum methods offer significant advantages in terms of both reduced cost and high throughput manufacturing capability. Electrodeposition or electroplating of metals (from metals dissolved in solution) onto conductive substrates represents an alternative CIGS fabrication strategy. Finally, CIGS fabrication via deposition of mixed binary, ternary, and/or quaternary nanoparticles of copper, indium, gallium, and selenium (nanoparticle “inks”) embodies another non-vacuum approach.
In addition to the approaches described above, a number of alternative approaches and hybrid strategies have been reported with varying degrees of success. Overall, CIGS fabrication via solution-based approaches appears to offer a convenient, low-cost option. According to this method, metal salts or metal complexes (precursors) of copper, indium, and gallium are dissolved in a solvent to form a CIGS ink and subsequently deposited on a substrate to form a film using conventional methods.
Mitzi et al. described a solution-based CIGS absorber layer deposition strategy using homogenous solutions of Cu, In, Ga and Se (and optionally sulfur) obtained by dissolution in hydrazine without the requirement for post-deposition, high-temperature selenization.1-3 Subsequently, a hydrazine-free approach was reported whereby isolated hydrazinium-based precursors could be deposited to form metal chalcogenide composite films.4 More recently, Mitzi et al. described a 15.2% PCE for a thin-film solar cell with a solution-processed Cu(In,Ga)(S,Se)2 (CIGS) absorber, which represents the highest reported value for a pure solution deposition method.5 With the exception of the aforementioned cases, solution-based deposition methods for fabricating a CIGS absorber layer have historically provided significantly lower performance relative to vacuum and electrodeposition processes.
Keszler et al. described a solution-based approach for the synthesis of low contamination metal chalcogenides in aqueous media.6 In this case, the formulations consist of aqueous solutions of metal chalcogenide precursors as a mixture of metal cation salts, formate anions and a source of chalcogenide (selenium, sulfur) in the form of thermally labile precursors including thiourea, thioformamide, selenourea, selenoformamide, etc. Overall, this method offers both environmentally favorable processing and low CIGS film contamination due to the careful selection of appropriate precursor materials. Finally, Wang et al. reported an inkjet printing method whereby the CIGS absorber layer was printed on molybdenum (Mo)-coated substrates from a solution of Cu, In, and Ga materials containing ethylene glycol and ethanolamine.7 Following selenization and CIGS device integration, an overall PCE of 5.04% was obtained. Subsequently, Wang et al. demonstrated CIGS solar cell performance exceeding 8% through careful optimization of Cu, In and Ga precursor formulations.8 As previously mentioned, solution processing methods for CIGS generally suffer from lower performance relative to conventional vacuum (and/or electrodeposition) approaches, with the exception of the hydrazine-based method. In response to this, appropriate impurity doping strategies offer the potential to dramatically improve the morphology and PV behavior of the CIGS absorber composite, thereby decreasing the performance “gap” between solution and vacuum processes for CIGS. Fortunately, CIGS exhibits a robust tolerance towards defects and/or impurities. In light of this, benefits from impurity doping have been successfully exploited through improvements in CIGS layer morphology that ultimately translate into better device performance primarily through a reduction of grain boundaries, which suppresses recombination phenomena. Yuan et al. provided CIGS solar cells with the incorporation of antimony (Sb) into a hydrazine-based method.9 Through this strategy using a 1.2 mol % Sb doping level, PCEs of 10.5% and 8.4% were achieved for CIGS solar cells processed at 400° C. and 360° C., respectively.10 A parallel study described an optimized method for Sb doping in a hydrazine-based approach with Sb dopant levels in the 0.2-1.0 mol % range.11 In this case, a PCE exceeding 12% for CIGS solar cells featuring Sb dopant was demonstrated. Unfortunately, the high toxicity and reactivity associated with hydrazine is a major disadvantage which may limit the practical adoption of these approaches in large-scale production environments.
Yatsushiro et al. investigated the impact of Sb inclusion on CIGS thin-films and solar cells.12 In this case, an Sb layer (˜10-50 nm) was initially deposited onto Mo-coated soda lime glass (SLG) and SiOx-coated SLG. In the latter case, the SiOx film functioned as a barrier to Na diffusion from SLG in order to isolate the impact of Sb on CIGS performance. Next, CIGS thin films were deposited on top through a three-stage process using molecular beam epitaxy (MBE). Overall, an enhancement in CIGS grain growth was observed for Sb-doped CIGS when SLG substrates were used although not similarly in the case when an alkali diffusion barrier on SLG was employed, leading the authors to assert that the beneficial doping of CIGS with Sb is limited to those scenarios where Na is also present in the CIGS layers.
In contrast to solution-based approaches for CIGS fabrication, Aksu et al. described a method for electrodeposition of a group VA metal including antimony (Sb), arsenic (As) and bismuth (Bi) into an electroplated metal composite, which is applicable to the fabrication of absorber materials (CIGS, for example).13 Specific aspects of the technology include an electroplating solution for deposition of a thin film that includes a Group VA material and a method of electroplating for depositing a thin film that includes a Group VA material.    1. D. B. Mitzi, M. Yuan, W. Liu, A. J. Kellock, S. J. Chey, V. Deline and A. G. Schrott, “A High-Efficiency Solution-Deposited Thin-Film Photovoltaic Device”, Advanced Materials 2008, 20, 3657-3662.    2. D. B. Mitzi, M. Yuan, W. Liu, A. J. Kellock, S. J. Chey, L. Gignac and A. G. Schrott, “Hydrazine-Based Deposition Route for Device-Quality CIGS Films”, Thin Solid Films 2009, 517, 2158-2162.    3. D. B. Mitzi, W. Liu and M. Yuan, “Photovoltaic Device with Solution-Processed Chalcogenide Absorber Layer”, US2009/0145482 A1.    4. D. B. Mitzi and M. W. Copel, “Hydrazine-Free Solution Deposition of Chalcogenide Films”, U.S. Pat. No. 8,134,150 B2.    5. T. K. Todorov, O. Gunawan, T. Gokmen and D. B. Mitzi, “Solution-Processed Cu(In,Ga)(S,Se)2 Absorber Yielding a 15.2% Efficient Solar Cell”, Progress in Photovoltaics: Research and Applications 2012, doi:10.1002/pip.1253.    6. D. A. Keszler and B. L. Clark, “Metal Chalcogenide Aqueous Precursors and Processes to Form Metal Chalcogenide Films”, US2011/0206599 A1.    7. W. Wang, Y-W. Su and C-H. Chang, “Inkjet Printed Chalcopyrite CuInxGa1-xSe2 Thin Film Solar Cells”, Solar Energy Materials & Solar Cells 2011, 95, 2616-2620.    8. W. Wang, S-Y. Han, S-J. Sung, D-H. Kim and C-H. Chang, “8.01% CuInGaSe2 Solar Cells Fabricated by Air-Stable Low-Cost Inks”, Physical Chemistry Chemical Physics 2012, 14, 11154-11159.    9. M. Yuan, D. B. Mitzi and W. Liu, “Techniques for Enhancing Performance of Photovoltaic Devices”, US2009/0320916 A1.    10. M. Yuan, D. B. Mitzi, O. Gunawan, A. J. Kellock, S. J. Chey and V. R. Deline, “Antimony Assisted Low-Temperature Processing of CuIn1-xGaxSe2-ySy Solar Cells”, Thin Solid Films 2010, 519, 852-856.    11. M. Yuan, D. B. Mitzi, W. Liu, A. J. Kellock, S. J. Chey, V. R. Deline, “Optimization of CIGS-Based PV Device Through Antimony Doping”, Chemistry of Materials 2010, 22, 285-287.    12. Y. Yatsushiro, H. Nakakoba, T. Mise, T. Kobayashi and T. Nakada, “Effects of Antimony Doping on Cu(In1-x,Gax)Se2 Thin Films and Solar Cells”, Japanese Journal of Applied Physics 2012, 51, 10NC25-1 to 10NC25-4.    13. S. Aksu, S. Lastella and M. Pinarbasi, “Electrodepositing Doped CIGS Thin Films for Photovoltaic Devices”, US20120214293.
It would be advantageous if a solution-based process existed for the fabrication of a Group VA-doped metal chalcogenide, such as an antimony-doped Cu—In—Ga—Se (CIGS) composite material.